(A) says, "that the sales department taken by itself is not critical
to the company's success as a whole"
The issue with (A) is that the shipping manager doesn't argue with the
sales manager's evidence; rather, he's simply saying that the sales
manager's evidence leads to an unsupportable conclusion.
(D) says, "an absurd consequence of its apparent assumption that a
department's necessity earns it the highest priority"
(D) looks great. The shipping manager points out that it's absurd to
think that more than one department could have the highest priority.
You can't give them all the highest priority; you can only assign the
highest priority to one of them, regardless of how important they all
are.